The easiest way to organize Kindle books is by placing them into collections. Most people base their collections around authors or genres. However, you could also create collections based on your favorite titles, books you have yet to read, or any other categories that make sense to you.
The following tips should help you create and manage your collections 
1. From the Home screen, choose "Create New Collection" from the menu. Name your collection according to how you want to organize your books. For instance, if you want to organize them by author you may have a collection titled "Anne Rice" and another titled "Dean Koontz", etc. If, on the other hand, you want to organize them by genre, you could create collections such as "Mysteries" or "Romance Novels".
2. After you create a collection, a list of books that can be added to the collection will appear. Select the checkbox next to each of the books that you want to add to that collection. When you have finished, press the "Done" button.
3. To add a book to a collection after the fact, simply press and hold its title until a menu appears. From this menu, choose "Add to Collection". A list of your collections will appear. Select the checkbox next to the collection that you want to add the book to and then press "Done".
4. To remove a book from a collection, simply press and hold the collection title from the Home screen. A list of items in that particular collection will appear. Deselect the checkbox next to any of the titles that you want to remove from the collection. Alternatively, from within a particular collection, you can tap on the menu and select "Add/Remove Items" from the drop-down menu. From there, simply uncheck the boxes next to the items that you want to remove.
Deleting a collection will not delete the books that are stored inside of it. Instead, the books will simply be listed directly on your Home screen rather than being associated with any collections.
In the past, this only worked for books that you had purchased outright from Amazon.  However, they recently changed it so that you can add books that you have borrowed with Kindle Unlimited or from the Kindle Owners' Lending Library to collections as well so that you can easily organize all of your content.
Here's a video that walks you through the process of creating a new collection and adding or deleting content from it: